Atm no one can replace Babar in the Test because there is no replacement, despite his average performance as captain.
Pakistan was a favorite against SL but they messed up with their selection by going with extra seamer instead of Spin. The Series against Australia was an embarrassment due to the flat deck but that's PCB's fault.
I dont know if there is actually any player who can replace Babar Azam as captain of Pakistan. I know that a captain has to be somewhat different from a player in terms of thinking. I think Babar Azam has that ability. Its the Pakistan players who are not performing consistently. Thats why Pakistan is losing or underperforming.
Pakistan should try to play to their strengths. Pakistan should always think about which part they are proficient at and not worry about exploiting the weakness of the opponent too much because that can create some confusion. If Pakistan makes another flat deck again it is going to be a big surprise.
If I am being honest then i find him a very defensive captain in the test. His batters are not performing is not his fault but fielding and bowling changes are part of the Skipper's job.
I've noticed during the field, most of the time he opts for a conservative approach despite being in a strong position.
BTW the last SL vs Pak bilateral series is under the scope due to a match-fixing allegation made by Srilakan's parliamentarian. ICC anti corruption unit might start their investigation soon.